Nowadays, in the fields of, for example, automobile, domestic electric appliance, and building material industries, there is an increasing demand for a high-strength steel strip (high tensile strength steel strip) capable of contributing to, for example, the weight reduction of structures. In the case of a technique using this high tensile strength steel strip, it may be possible to manufacture a high-strength steel strip having good stretch flange formability by adding Si in steel. In addition, in the case of a technique using this high tensile strength steel strip, it may be possible to provide a high-strength steel strip having good ductility due to a tendency for a retained γ phase to be formed by adding Si and Al in steel.
However, in the case of a high-strength cold-rolled steel strip containing easily oxidizable metals such as Si and Mn, there is a problem in that these easily oxidizable metals are concentrated in a surface portion of the steel strip when annealing is performed and oxides of, for example, Si and Mn are formed, which results in surface appearance defects or defects in a chemical conversion treatment such as a phosphating treatment.
In addition, in the case of a galvanized steel strip containing easily oxidizable metals such as Si and Mn, there is a problem in that these easily oxidizable metals are concentrated in a surface portion of the steel strip when annealing is performed and oxides of, for example, Si and Mn are formed, which results in nonplating defects due to a decrease in zinc coatability or results in a decrease in alloying speed when an alloying treatment is performed after a coating treatment has been performed.
In particular, in the case where Si is contained and an oxide film of SiO2 is formed on the surface of a steel strip, there is a significant decrease in wettability between the steel strip and a molten coating metal. In addition, the oxide film of SiO2 functions as a barrier to diffusion between the base steel and a coating metal when an alloying treatment is performed, which results, in particular, in a problem of a decrease in zinc coatability and alloying treatment performance.
As an example of a method for avoiding these problems, consideration is given to a method for controlling the oxygen potential in an annealing atmosphere.
Patent Literature 1 discloses an example of a method for increasing the oxygen potential in which the dew point is controlled to be high, that is, −30° C. or higher from a rear heating zone to a soaking zone. This method can be expected to be effective to some extent and has an advantage in that the dew point can be controlled to be high easily in an industrial manner.
However, this method has a disadvantage in that, with this method, it is not easy to manufacture some steel grades (such as Ti-based IF (Interstitial Free) steel) for which an operation in an atmosphere having a high dew point is not desirable. This is because it takes a very long time to control the dew point of an annealing atmosphere to be low once the dew point has been controlled to be high. In addition, since an oxidizing furnace atmosphere is used in this method, there may be a problem of pickup defects due to oxides sticking to rolls in the furnace and of furnace wall damage in the case where there is a control error.
As another example, consideration is given to controlling the oxygen potential to be low.
However, in the case of a large-scale continuous annealing furnace which is used in a CGL (continuous galvanizing line) or a CAL (continuous annealing line), since Si and Mn are very easily oxidized, it is very difficult to stably control the dew point of the furnace atmosphere to be low, that is, −40° C. or lower where there is a good effect for suppressing oxidation of, for example, Si and Mn.
Although Patent Literature 2 and Patent Literature 3 disclose techniques with which an annealing atmosphere having a low dew point can be efficiently achieved, since these techniques are intended for comparatively small-scale furnaces of a one-pass vertical type, no consideration is given to annealing a steel strip containing easily oxidizable metals such as Si and Mn by using an annealing furnace of a multipass vertical type such as a CGL or a CAL.